One major component of all wireless transmitters is a power amplifier. It is well known that power amplifiers are non-linear devices. The non-linearity of a power amplifier manifests itself in the form of inter-modulation distortion products, which lie very close to the transmitted signal in the frequency domain. These products cannot be removed by simple filtering. As such, alternative techniques, such as predistortion, have emerged to compensate for the non-linearity of power amplifiers.
Another type of distortion caused by power amplifiers, particularly when amplifying high bandwidth signals, is phase distortion resulting from memory effects of the power amplifier. Memory effects are defined as effects that occur when an output sample of the power amplifier is not only a function of a current input sample of the power amplifier but also past input samples of the power amplifier. The result of the phase distortion caused by the memory effects is generally seen as an asymmetric response in the inter-modulation products in frequency bands adjacent to the desired frequency band.
Memory effects are generally classified as short-term memory effects and long-term memory effects. Short-term memory effects are generally due to mismatches in the biasing network of the power amplifier, electro-thermal distortions, etc. These effects are generally in the order of 10 to 50 nanoseconds. Long-term memory effects on the other hand can occur due to phenomena such as charge trapping in the semiconductor lattice, which can result in slow steady state stabilization of the drain current. Typically, long-term memory effects are on the order of milliseconds or more.
There is a need for a system and method that provides power amplifier predistortion to compensate for the non-linearity of the power amplifier as well as short-term and long-term memory effects.